Lymphocytes which are responsible for cell-mediated immune and humoral responses, can be divided into subpopulations on the basis of surface antigens and functional tests, but can not be differentiated by ordinary light microscopy. This study based upon computer analysis of objective image descriptors will differentiate peripheral and thoracic duct lymphocytes from normal mice and mice infected with various types of murine leukemia viruses. Objective image descriptor values which differentiate subpopulations of T and B cells, subsets of T cells, activated and non-activated lymphocytes, leukemic and normal lymphocytes, and lymphocytes responding to a leukemic process will be ascertained. Such objective image descriptors will provide for the establishment of a limited data bank to differentiate lymphocytes, which can not be differentiated by ordinary light microscopy, into subpopulations and subsets, to compare feature vectors of stimulated lymphocytes to feature vectors of lymphocytes functioning actively in immunosurveillance, and perhaps to detect feature descriptors of the leukemic cell before other signs of the leukemic process are evident.